Today, aircraft transports huge amounts of goods. Although the reliability and safety concerning the shipments generally is good, a part of the goods will always be misrouted or stolen. Many shipping companies provide various types of additional shipping service for goods and materials. One possibility is to place tracking information on the container housing the shipped goods so that the container can be tracked during shipment at various points. This allows the shipping company to determine the last geographic location of the container as it moves between the origination and destination points and to monitor whether the goods inside the container are on time, late, or somehow misplaced. However, such systems still rely on manual operations to certify the shipment.
In order to provide a more continuous and reliable, containers can be provided with a tracking device. The tracking device automatically determines the position of the shipment container, intermittently or continuously. Preferably, the tracking device also comprises a remote communication equipment. This enables the tracking device to report the container position to a remote site. In such a way, the shipping company will be able to follow the shipment continuously and immediately detect any delays or misrouting. One example of such a tracking system is disclosed in the published international patent application WO 0175700.
Most remote communication equipments are based on communication via electromagnetic signals in the radio frequency range. Unfortunately, emission of radio frequency electromagnetic signals may interfere with vital systems in the aircraft, such as navigation and communication systems. In order to insure undisturbed operation of such systems, different aviation administrations have issued restrictions on use of certain electronic devices emitting radio frequency electromagnetic signals on an aircraft during its operation.
In order to follow such restrictions, container tracking devices are provided with proximity detectors, which detects if the container is in the vicinity of a transport vessel, in particular an aircraft. If the proximity to an aircraft is detected, the radio-frequency electromagnetic signal emitters in the tracking devices are deactivated, in order not to disturb the aircraft system. There are many different possible proximity detectors. Some of them are described in U.S. Pat. No. 6,281,797.
The most attractive proximity detector approach is to detect the occurrence of a 400 Hz electromagnetic field. Aircrafts in general has a main electrical distribution system that operates with a higher frequency—around 400 Hz—than used in a normal domestic power system. The main electrical distribution system gives rise to an emission of an electromagnetic field, which radiates out throughout the aircraft. If a container is brought into vicinity of a powered aircraft, the proximity detector will detect a 400 Hz electromagnetic field. In response to such detection, the radio-frequency electromagnetic field emitting devices, e.g. the radio communication means, are deactivated.
When the container is brought out from the aircraft, the 400 Hz electromagnetic field will disappear again, and the radio communication can be re-established.
A problem with this approach appears in cases where the main power system of the aircraft ceases to operate during a flight. In such a case, the power supply of the aircraft functions switches over to a battery operation. This battery backup is sufficient to supply the vital functions until the aircraft reaches the closest airport. Emissions from a direct current power system are uniform and cannot be distinguished. A prior-art proximity detector will in such a case notice a disappearance of the 400 Hz electromagnetic signal and will interpret this as if the container has left the a cargo room of the aircraft. Radio-communication from the tracking device will be allowed, which may disturb the aircraft systems, which already are put into an emergency operation.